The dual frequency radio signals of the Global Positioning System (GPS) allow measurements of the total number of electrons, called total electron content (TEC), along a ray path from GPS satellite to receiver. We have developed a new technique to construct two-dimensional maps of absolute TEC over Japan by using GPS data from more than 1000 GPS receivers. A least squares fitting procedure is used to remove instrumental biases inherent in the GPS satellite and receiver. Two-dimensional maps of absolute vertical TEC are derived with time resolution of 30 seconds and spatial resolution of 0.15° × 0.15° in latitude and longitude. Our method is validated in two ways. First, TECs along ray paths from the GPS satellites are simulated using a model for electron contents based on the IRI-95 model. It is found that TEC from our method is underestimated by less than 3 TECU. Then, estimated vertical GPS TEC is compared with ionospheric TEC that is calculated from simultaneous electron density profile obtained with the MU radar. Diurnal and day-to-day variation of the GPS TEC follows the TEC behavior derived from MU radar observation but the GPS TEC is 2 TECU larger than the MU radar TEC on average. This difference can be attributed to the plasmaspheric electron content along the GPS ray path. This method is also applied to GPS data during a magnetic storm of September 25, 1998. An intense TEC enhancement, probably caused by a northward expansion of the equatorial anomaly, was observed in the southern part of Japan in the evening during the main phase of the storm.

/pdf/a-new-technique-for-mapping-of-total-electron-content-using-3p7zuf6qai.pdf

A new technique for mapping of total electron content using GPS network in Japan

In May 1974 a new era in satellite radio beacon studies of the ionosphere opened with the ATS-6 Radio Beacon Experiment. The history of radio beacon studies up to that time is reviewed briefly and the particular features of the ATS-6 beacon are discussed together with the basic theory required to interpret the measurements. The main emphasis is on the ATS-6 beacon experiment but other beacon data are discussed which provide the necessary background. The diurnal and seasonal variations of the total electron content and the plasmaspheric content are presented for the U.S.A. and Europe. In winter the plasmaspheric content over the Western Hemisphere maximizes at night while in Europe and the Pacific it appears to peak near noon. This is thought to be caused by flow of plasma from the local and conjugate ionospheres. Night maxima of total electron content are found showing that they do not arise from depletions of the plasmaspheric content. The plasmaspheric content is highly sensitive to solarterrestrial disturbance, it reaches a minimum on the third day of a storm and may take between 10 and 20 days of partial filling and emptying to recover. Traveling disturbances in U.S.A., Europe, and India show similarities of speeds but not of direction. Beacon observations of micropulsations in total content, tropospheric fluctuations and Fresnel diffraction by intense ionospheric irregularities are discussed together with radio wave scintillations and some applications of beacon radio data to communications and navigation.

Recent progress in satellite radio beacon studies with particular emphasis on the ATS-6 radio beacon experiment

With the increasing number of precise navigation and positioning applications using Global Navigation Satellite Systems (GNSS) such as the Global Positioning System (GPS), higher order ionospheric effects and their correction become more and more important. Whereas the first-order error can be completely eliminated by a linear combination of dual- frequency measurements, the second- and third-order residual effects remain uncorrected in this approach. To quantify the second-order residual effect, a simple formula has been derived for GNSS users in Germany. Our proposed correction algorithm reduces the second-order effects to a residual error of fractions of 1 mm up to 2 mm at a vertical total electron content level of 1018 electrons/m2 (100 TECU), depending on satellite azimuth and elevation angles. The correction formula can be implemented in real-time applications as it does not require the knowledge of the geomagnetic field or the electron density distribution in the ionosphere along the signal path. It is expected that the correction will enable more accurate positioning using the line-of-sight carrier-phase measurements.

Higher order ionospheric effects in precise GNSS positioning

[1] First observations of intense GPS L1 amplitude scintillation activity in the midlatitude ionosphere at latitudes corresponding to the northeastern United States have been made. Moderate to severe, these scintillations result from space weather effects due to a disturbed ionosphere. Moderate to severe scintillations can degrade or even disrupt communication and navigation systems relying upon transionospheric radio wave propagation. A modified GPS receiver was used to record GPS satellite signal strength at Cornell University (53.2° magnetic latitude) during a magnetospheric disturbance on September 25–26, 2001 from 0000–0400 UTC. This disturbance (Kp = 6, minimum Dst = −110 nT) prompted the ionospheric trough to move equatorward over the northeastern U.S. and produced large plasma densities and gradients attributed to storm-time effects. This disturbance resulted in intense L-band amplitude scintillations (≥20 dB, S4 ≈ 0.8) which are highly uncharacteristic at this magnetic latitude. Concurrent measurements of TEC showed steep density gradients (∼30 TEC/deg) and evidence of irregularity structuring.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2002GL014770

First observations of intense GPS L1 amplitude scintillations at midlatitude

Severe disturbances of VHF and GHz waves from geostationary satellites during a magnetic storm

Propagation characteristics of gigahertz and very high frequency waves through the mid-latitude ionosphere have been studied applying the coherent beacon signals of 1.7, 11.5, and 34.5 GHz and the 136-MHz telemetry signal, which are emitted from the geostationary Engineering Test Satellite Type II (ETS-II). Total electron content (TEC) was obtained by measuring the differential phase shift between 1.7 and 11.5 GHz waves and also by the Faraday rotation of the 136-MHz carrier, the former method being more sensitive than the latter one for detecting fluctuating TEC. During the observations, amplitude scintillations at 136 MHz and 1.7 GHz, usually accompanied with an irregular variation of TEC, were recorded frequently in the nighttime and sometimes in the daytime. Specially noted is the 11.5-GHz scintillations less than 0.8 dBp-p that were observed during the severe magnetic storm on February 15, 1978, simultaneously with 136-MHz and 1.7-GHz scintillations. This was the first observation showing that the disturbed ionosphere affects waves above 10 GHz. Presented first are the techniques for the ETS-II ionospheric wave propagation experiment, including a description of the coherent receiver system in the gigahertz frequency range at Kashima and a description of the 136-MHz Faraday rotation receiver at Kokubunji. The capabilities of the facilities for monitoring the mid-latitude ionosphere by giving some observational results are then presented.

Jun. Awaka

Papers

Severe disturbances of VHF and GHz waves from geostationary satellites during a magnetic storm

ETS-II Experiments Part V: Effect of Ionosphere